1. Field of the Invention
The present invention relates to cryptographic communication method, encryption algorithm shared control method and network communication system for converting encryption algorithm for cryptographic communication to other encryption algorithm, and more particularly to cryptographic communication method, encryption algorithm shared control method and network communication system suitable for sharing the same encryption algorithm as encryption algorithm operated by a plurality of users and changing the shared encryption algorithm to other encryption algorithm.
2. Description of Related Art
As a means for securing the safety of information transmission, a method of transmitting encrypted information is generally employed. Because of recent advancement of performance of a personal computer, in a case where information to be transmitted is digital information such as document and video, often such information is encrypted on software basis.
If a user U[A] carries out cryptographic communication with a user U[B], the user U[A] encrypts transmission information with an encryption key and transmits that encrypted data. On the other hand, the user U[B] receives this data and decrypts that received data with a decryption key. This cryptographic communication can be established on a presumption that the user U[A] and user U[B] share the same encryption algorithm. Usually, the encryption algorithm is shared by the following manners. The encryption algorithm is recorded in a recording medium such as a floppy disk by an encryption system manager and distributed to each user. Or the encryption algorithm is installed in an information processing unit having encryption processing function such that it can be executed and the information processing unit is distributed.
As for a method for operating the encryption algorithm, to improve the cipher security, not only a scramble key is generated as a key for encrypting information, but also a session key for encrypting this scramble key is generated. Then, duplex encryption method is employed so that user U[A] transmits information encrypted with the scramble key and the scramble key encrypted with the session key to user U[B]. Each time when cryptographic communication occurs, the scramble key is changed.